Gas turbine engines and other turbomachines have rows of blades which rotate within a generally cylindrical case. As the blades rotate, their tips move in close proximity to the internal wall surface of the case. To maximize engine operating efficiency, leakage of the gas or other working fluid between the blade tips and the case should be minimized. As has been known for some time, this may be achieved by blade and seal systems in which the blade tips rub against a seal attached to the case interior. Generally, the blade tip is made to be harder and more abrasive than the seal so that the tip cuts into the seal during those portions of the engine operating cycle when they contact each other.
Abrasive blade tips which are particularly useful in the high temperature section of gas turbine engines are described in commonly assigned U.S. Pat. Nos. 4,249,913 to Johnson et al, entitled "Alumina Coated Silicon Carbide Abrasive" and 4,610,698 to Eaton et al entitled "Abrasive Surface Coating Process for Superalloys". The contents of both patents are incorporated by reference. According to the Johnson et al invention, silicon carbide abrasive particles of about 200-750 microns (8-30 mils) average diameter are coated with a metal oxide such as alumina and incorporated by powder metal techniques in nickel or cobalt base matrix alloys. A powder metal compact containing up to about 45 volume percent of these ceramic particles may be made which is then bonded to the tip of the blade. According to the Eaton et al invention, a single layer of alumina coated silicon carbide abrasive particles, having a metal coating thereon, are first disposed on the blade tip surface in spaced apart relation, and then sintered to the blade tip surface by heating to a high temperature. A matrix material is then applied over the particles by plasma spraying. According to Eaton, the metal coating on the particles is deposited by an electroless nickel plating process.
Notwithstanding the advances described in these two patents, further improvements are needed in order to fabricate abrasive layers used on blades which operate in advanced turbine engines. One area in which improvement is sought relates to methods for increasing the bond strength between the abrasive particles and the blade tip. When the abrasive layer is made in accordance with the Eaton patent, a high strength bond is especially needed during the application of the plasma sprayed matrix layer.